The present invention relates to a color copier, color printer, color facsimile apparatus or similar color image forming apparatus.
A color image forming apparatus of the type leaving a photoconductive belt and an intermediate image transfer belt is conventional. It has been customary with this type of apparatus to provide each belt with a mark or a notch and cause a photosensor or similar sensing member, mounted on the apparatus body, to sense it. This allows position information relating to the belt to be detected.
Japanese Patent Laid-Open Publication No. 4-335665, for example teaches a color image forming apparatus including a photoconductive belt provided with a triangular mark, and mark sensing means for sensing the mark with a laser beam. The mark sensing means senses the mark every time the belt is exposed by a light beam associated with an image forming signal of one color component. The rotation phase of a polygonal mirror and, therefore, the time for starting exposure in the subscanning direction is controlled on the basis of the time when the mark sensing means senses the mark. This kind of scheme is intended to produce a color image free from color displacement. The apparatus sequentially transfers toner images of different colors from the photoconductive belt to an intermediate image transfer belt one above the other, and then transfers the resulting composite image from the intermediate belt to a paper at a time. However, the monocolor images are necessarily displaced from each other because the circumferential length of the intermediate belt and the scanning interval of the polygonal mirror do not have an integral multiple relation Theoretically, therefore, one line of displacement occurs between the consecutive monocolor toner images.
Japanese Patent Laid-Open Publication No. 61-34162 discloses an arrangement wherein a mark for sensing the seam of a photoconductive belt is formed on the belt by the thermal transfer of ink from a thermally fusible ink layer. However, the thermal transfer method is practicable only with limited substances. For example, an intermediate image transfer member made of fluorine-contained resin, e.g., ETFE does not allow the ink to be transferred thereto because it has a non-adhesive surface. Moreover, even when the belt allows the ink to be transferred thereto, the transfer at high temperature is likely to affect the property of the belt.
Further, Japanese Patent Laid-Open Publication No. 60-134274 proposes a photoconductive belt formed with notches for timing detection at opposite edges thereof. The photoconductive belt, having a base made of PET (polyethylene telephthalate), scarcely cracks at the notches or otherwise deteriorates despite the notches. However, the intermediate belt, made of ETFE or similar fluorine-contained resin, would crack at the notches during the course of repeated rotation. Specifically, because the mechanical strength depends on the material, the notch scheme is limited by the material of the belt and not suitable for the intermediate belt.
The intermediate belt made of ETFE exhibits a far more noticeable non-adhesion characteristic than belts made of the other materials. Therefore, it is difficult to form a mark on the ETFE belt by hot stamping, painting, printing or similar technology. Although the mark itself may be adhered to the belt by, for example a two-sided adhesive tape, it is likely that the mark is turned up and disturbs the operation of a photosensor or similar sensing means. In an color image forming apparatus of the type using such an intermediate belt and controlling the registration of colors on the basis of a mark provided on the belt, the contamination, turn-up and other disfigurement of the belt cause the colors to b e displaced from each other and results in irregular colors.